Changes of state

GAS > LIQUID

This is called condensing / condensation. (heat) Energy is removed.

Solid > Liquid

Energy is added - heat is applied. Some elements / compounds will (of course) require more heat than others. The more heat is required, the stronger the bonds between atoms.

Liquid > Solid

Heat (energy) is removed

Liquid > Gas

In both cases, energy (heat) is added. However, evaporation and boiling aren't the same thing!

• Boiling is when all the liquid turns into a gas within a short time frame (i.e. putting the kettle on until all the water is gone)
• Evaporation is when liquid slowly turns into a gas. The particles at the top have more access to the heat source (typically the sun) and become the gas, while cooler particles are left behind. While eventually, all the liquid could turn into a gas, it would take a very, very long time.

Solid > Gas

This is called sublimation. Energy is added. An example is dry ice, which sublimes immediately.